Flank marking, a form of olfactory communication in Golden hamsters (Mesocricetus auratus) is dependent upon a vasopressin-sensitive pathway in the medial preoptic area (MPOA) of the hypothalamus. Vasopressin (VP) microinjected into a discrete site of the MPOA reliably elicits intense bouts of flank marking and an antagonist to vasopressin injected into the same site inhibits flank marking induced by microinjection of VP or the odors of other hamsters. Studies have been proposed to define the afferent and efferent connections of the area within the MPOA controlling flank marking behavior by horseradish peroxidase histochemistry. Once identified, these sensory and motor connections will be lesioned to determine their relevance in the expression of flank marking. The concentration of VP within the effective site of the MPOA will be determined by chromatographic and immunochemical methods and the binding characteristics of VP within this site will be determined by Scatchard analysis. Various analogs and metabolites of VP will be microinjected into the MPOA to study the structure-activity relationship between VP and flank marking. Finally, a series of studies will be conducted to investigate the function of flank marking behavior in specific social situations (i.e. aggressive and reproductive interactions). In summary, vasopressin-dependent flank marking in the MPOA provides an excellent model system for studying the neuroanatomy and chemistry involved in the organization and expression of a complex communicative behavior. Defining the anatomical and neurochemical bases of a mammalian behavior should eventually lead to a better understanding of normal and abnormal human behavior.